The datetime strings use the ISO 8601 notation. An UTC datetime string looks like 2023-01-11T12:00:00.000Z, Z is the zone designator for the zero UTC offset. Without the Z it would be seen as a local time, with local times the T is often replaced with a space like 2023-01-11 12:00:00.000.
When you use one of the date formatters on the client it will use the local timezone, when the datetime string didn’t contain timezone information it will be handled as if it were the local timezone. With UTC datetime strings (or any other timezone offset) it will convert it to the timezone of the client.